In electrostatic copying, an electrostatic latent image is formed on an element. That image can be developed into a visible image by the application of toner powder thereover. The resulting toned image is then transferred from the element to a receiver to which the transferred toned image is fixed, usually by heat fusion. The transfer of the toned image to the receiver has usually heretofore been accomplished electrostatically, using an electrostatic bias applied between the receiver and the element.
In order to produce copies of very high resolution, it is necessary to use toner particles that have a very small particle size, that is, less than about 8 microns.
Electrostatic transfer of such very small toner particles, particularly of those having a particle size less than about 6 microns, is difficult to accomplish because, the forces of adhesion holding the particles to the photoconductor are greater than the electrostatic transfer forces that can be applied. Moreover, Coulombic repulsion between the particles tends to scatter such particles causing loss in transferred image resolution and increase in grain and mottle. To avoid these problems a non-electrostatic transfer process must be used with toned images of such particles.
One suitable such transfer process is provided by the teaching of copending U.S. patent application Ser. No. 230,394 filed Aug. 9, 1988 wherein a thermally assisted transfer procedure is utilized. A receiver is heated, so that, in the transfer nip the temperature typically is in the range of about 60.degree. to about 90.degree. C., and is contacted against the toned image formed on the element. The heated receiver sinters the toner particles, causing them to stick to each other and to the receiver, thereby effecting a transfer of the toned image from the element to the receiver. The element and the receiver are separated, while still hot. Subsequently, the transferred toned image is fused to the receiver. This process is sometimes called the thermally assisted transfer process. This process is useful, but suffers from the disadvantage that, while scattering is avoided, some toner particles may not transfer from the element. Moreover, it is frequently necessary to add a release agent to the photoconductor. The use of a release agent can be avoided if a receiver bearing a thermoplastic resin is used. However, this restricts the types of receivers that can be used.
However, so far as now known, no thermally assisted transfer process is known by which a high resolution toner powder image comprised of very small toner particles can be transferred from an element to a receiver that is treated at the time of copying with a preliminary coating produced from toner particles.